In this work, we analyzed the experimental pressure-temperature behavior in the Isotropic-Nematic phase transition for the liquid crystal p-azoxianisol at 1 atm using a development for the HERSW Conveg Peg model. Additionally, we obtained the values of the molecular volumes for the hard and attractive cores from theoretical quantum calculations at PM3, PM6 and B3LYP/6-311++G levels considering the molecular interaction among the liquid crystals (PAA)(5). We found that the best prediction for experimental data appears when the effect of the molecular interaction is considered in the volume calculation.